Field of the Invention
The present invention relates to the field of gas turbine engines for propelling an aircraft.
Description of the Related Art
In the following, the terms “upstream” and “downstream” are defined in relation to the flow direction of the air flow in the engine. A gas turbine engine conventionally comprises, from upstream to downstream, a low-pressure compressor, a high-pressure compressor, a combustion chamber, a high-pressure turbine and a low-pressure turbine. To this end the engine comprises a low-pressure spool supporting compressor and turbine rotor blades and a high-pressure spool supporting compressor and turbine rotor blades. Upstream, the low-pressure compressor comprises, in a known manner, large rotor blades known to a person skilled in the art as a “fan”.
One of the ways to limit the fuel consumption of the engine is to reduce the rotational speed of the fan blades in order to increase the bypass ratio of the engine, that is to say, the ratio between the air mass of the bypass flow (air mass displaced by the fan and not passing through the combustion chamber) and the air mass of the hot flow (air mass passing through the combustion chamber). As indicated above, the fan blades and the low-pressure turbine rotor blades are rigidly connected to the low-pressure spool of the engine. Consequently, a reduction in the speed of the fan blades brings about a reduction in the speed of the low-pressure turbine blades. Such a solution affects the performance of the turbine of the engine, and this is not desirable. In order to eliminate this drawback, it has been proposed to uncouple the rotation of the fan blades from the rotation of the low-pressure turbine blades such that their rotational speeds are different.
A first uncoupling solution consists in providing a torque reducer between the low-pressure turbine and the fan. Without mentioning its high mass, a torque reducer must dissipate a large amount of thermal energy, and this can have drawbacks in terms of reliability.
A second uncoupling solution consists in providing, in addition to the low-pressure and high-pressure spools, an intermediate spool which allows the speed of the fan blades, which are rigidly connected to the low-pressure spool, to be made independent of the speed of the low-pressure turbine rotor blades, which are rigidly connected to the intermediate spool. By definition, a spool comprises a shaft on which compressor rotor blades and turbine rotor blades are mounted. Each rotor blade of a spool is associated in the engine with a stator blade, mounted upstream of said rotor blade, the purpose of which is to straighten the air flow which has been deflected by the rotor blade. As shown in FIG. 1, a triple-spool engine comprises, in a known manner, a low-pressure rotary body 100, an intermediate rotary body 200 and a high-pressure rotary body 300, which respectively comprise compressor rotor blades 100C, 200C, 300C associated with compressor stator blades 101C, 201C, 301C and turbine rotor blades 100T, 200T, 300T associated with turbine stator blades 101T, 201T, 301T. Such an engine has a large mass and is very long, which has drawbacks in terms of dimensions and mass, and this limits fuel savings.